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Shear failure often occurs during the construction of the geotechnical engineering projects, especially under dynamic loading condition. To investigate the effect of loading rate and normal stress on the shear responses of rock materials, a bonded-particle model (BPM) was adopted to insight the failure process from a micro-mechanical perspective using the moment tensor. Firstly, we propose a novel calibration methodology for direct shear test, which provides a possibility to effectively derive microscopic parameters of PBM for obtaining the macroscopic properties of rocks considering loading rates under direct shear. Validation results show that the simulation demonstrated good agreement with the experimental results. Then, a series of simulated direct shear tests under different normal stresses ranging from 0 to 20 MPa and loading rate ranging from 10–3 to 10–1 mm/s were performed. The results demonstrate that both normal stress and loading rate have a positive influence on shear strength. However, normal stress has a greater impact on the shear strength compared to loading rate, and the effect of loading rate on shear strength weakens as the normal stress level increases. Increasing the loading rate and normal stress level led to a higher distribution of AE events in the nonlinear stage. At lower loading rates and normal stress, intense AE activity was concentrated around the peak stress point. In contrast, under higher situations, the nonlinear stage is prolonged, resulting in an increased number of AE events in the nonlinear and post-peak stages. Normal stress has a greater impact on the generation and propagation of micro-cracks compared to loading rate. With higher normal stress, the failure mode shifts from a shear-dominated to a mixed mode with tensile micro-cracks in a dominant role. Highlights: A numerical model was established using DEM to investigate the mechanical properties and failure mode evolution of rock specimens under shear loading, taking into account the influence of loading rate and normal stress factor. A novel calibration procedure was proposed for the direct shear model, and the results showed that it effectively improved the calibration efficiency compared with traditional methods, as validated by experimental data. The fracture behavior of the specimen under different strain rates and normal stress states was monitored using numerical simulation based on moment tensor theory of acoustic emission, which allowed for analysis of the shear failure process. [ABSTRACT FROM AUTHOR]

Some rock discontinuities, such as bedding planes, incipient and healed joints, can sustain significant tension. Rock masses are often subjected to dynamic loadings. However, the ways in which dynamic tension affects the properties of rock discontinuities remain poorly understood. To investigate the tensile response of rock discontinuities under high strain rates, direct tension tests using the split Hopkinson tension bar were conducted on natural limestones with discontinuities including stylolitic joints and mineral healed joints. The results demonstrate that the dynamic tensile strengths of these joints increase with increasing strain rate, and a nonlinear empirical relationship is proposed to capture these effects. In addition, the dynamic tensile stress sustained by the discontinuity shows a nonlinear relationship with its opening (i.e., displacement of the discontinuity under tension). The critical joint opening increases almost linearly with increasing strain rates. The findings of this study can enhance the understanding of the behavior of rock discontinuities under dynamic tension, and facilitate dynamic and seismic analyses for various rock engineering problems. Highlights: The split Hopkinson tension tests were conducted on rock discontinuity. Rock discontinuity deforms nonlinearly under dynamic direct tension. The dynamic direct tensile strength of rock discontinuity exhibits an obvious strain rate effect. [ABSTRACT FROM AUTHOR]

Understanding the dynamic cracking behaviors and energy evolution of flawed rocks is highly relevant to underground rock engineering. In this study, multi-flawed rock specimens are tested under coupled static–dynamic compression using a modified SHPB system combined with high-speed photography and DIC monitoring. We systematically investigated the influences of pre-stress ratio, flaw inclination angle and strain rate on the dynamic progressive cracking mechanism and energy evolution of multi-flawed rocks. Experimental results show that the dynamic/total strength generally increases with increasing strain rate, featuring evident rate-dependence. With increasing flaw inclination angle from 15° to 60°, the dynamic/total strength initially decreases and subsequently increases with the minimum achieved around 45°. With the pre-stress ratio increasing from 0.2 to 0.8, the dynamic strength persistently decreases while the total strength initially increases and subsequently decreases with the maximum achieved at 0.6. Furthermore, based on the displacement trend lines method, a novel crack classification method is developed to analyze the progressive cracking mechanism of multi-flawed rocks using high-speed photography and DIC technique. Generally, mixed cracking dominates the failure of multi-flawed rocks under coupled static–dynamic compression. With increasing flaw inclination angle form 15°–60°, the predominant cracking mechanism changes from mixed tensile-shear cracking to mixed compression-shear cracking. The increasing pre-stress ratio promotes shear cracking under lower flaw inclination angles while facilitates tensile cracking under higher flaw inclination angles. In addition, the energy evolution for coupled static–dynamic SHPB tests is re-evaluated and a new energy calculation formula is proposed. The results show that the increasing strain rate reduces the energy utilization while promotes the energy dissipation density. Both the energy utilization and energy dissipation density increase with increasing pre-stress ratios. [ABSTRACT FROM AUTHOR]

Keywords: 3D rock dynamics; True triaxial electromagnetic Hopkinson bar; Rock dynamic test EN 3D rock dynamics True triaxial electromagnetic Hopkinson bar Rock dynamic test 2079 2086 8 03/30/21 20210401 NES 210401 Introduction With increasingly more large rock structures built in regions where severe dynamic disturbance and complex tectonic stress present, the engineering disturbance induced dynamic disasters (e.g., rockburst, coal bump, landslide) are becoming more frequent and severe, resulting in huge casualties and property losses (Jiang et al. [9]; Linzer et al. [13]). [Extracted from the article]

Heterogeneous rock contains numerous pre-existing three-dimensional (3D) cracks, which control its mechanical and fracturing properties. Considerable effort has been devoted to studying the volumetric fracturing behaviour of rock under static loading conditions. Although rock masses are often subject to dynamic impacts such as earthquakes and blasting, the mechanical and volumetric fracturing behaviour of rock under dynamic loading is still poorly understood. In this paper, dynamic laboratory tests were performed on 3D-printed artificial rock samples with 3D embedded flaws created during three-dimensional printing (3DP), with the aim of studying the volumetric fracturing and mechanical properties of these samples under impact with high strain rate. The results show that the dynamic compressive strength and the tangent modulus decrease with an increasing number of flaws, but have very limited effects on the ratio of the fracture initiation stress of the first crack to the peak stress of the sample, the maximum axial strain of the sample and the volumetric fracturing behaviour of the sample. The tensile failure of a sample is caused by the continuous extension of wing cracks from the outer flaw tips. The mechanical and volumetric fracturing behaviour of samples with 3D embedded flaws are strain rate dependent. The tangential modulus and the ratio of the fracture initiation stress of the crack to the peak stress increase significantly when the loading type changes from static compression to dynamic compression. Under dynamic compression, wing cracks can continuously extend to the sample ends, whereas under static compression, wing cracks can intermittently extend only a limited distance. Moreover, the fracturing behaviour of 3D flaw differs from that of 2D flaws under dynamic loading. Under high strain rate loading, wing cracks generated at 3D flaw tips lead to splitting failure of the sample, while shear cracks formed at 2D flaw tips result predominant shear failure of the sample. The findings in this paper could facilitate a better understanding of rock failure subjected to dynamic loading conditions. [ABSTRACT FROM AUTHOR]

Mixed mode I/II fracture often occurs when rock with pre-existing defects is subjected to dynamic impact. However, the understanding of dynamic mixed mode I/II fracture of rock is still at its infancy. To investigate rate dependent behavior of mixed mode I/II fracture toughness, the split Hopkinson pressure bar tests were performed on cracked straight through Brazilian disc sandstone specimens with different loading angles and strain rates. The results show that the peak load and peak strain increase with increasing strain rates while the influence of loading angle is almost negligible. Dynamic mode I and mode II stress intensity factors (SIFs) as well as effective fracture toughness increase rapidly with the increase in strain rate before slow increase as the strain rate exceeds approximately 173 s−1. As the loading angle increases, the dynamic mode I SIFs gradually decreases, while the dynamic mode II SIFs first increases and then decreases. The dynamic fracture initiation toughness increases linearly with increasing strain rates. In addition, a novel methodology for quantitative analysis of rock microstructure was proposed to analyze quantitatively the strain rate effect on microstructure of the sandstone. It is found that with increasing strain rate, the area ratio of pores and microcracks on the fracture surface of sandstone increases, while the area ratio of transgranular fracture almost keeps constant. Pores and microcracks are firstly activated by the high-strain-rate loading, and then a complex interconnected crack system is formed, thereby enhancing the dynamic fracture toughness of sandstone. The findings of this paper could facilitate better understanding the rate effect of mixed mode I/II fracture and its micromechanism. [ABSTRACT FROM AUTHOR]

A deep understanding of how flawed rock cracks under high temperatures and dynamic impacts is crucial for safe deep rock engineering design and operation. Dynamic impact tests were conducted on sandstones with various flaw inclinations using a Split Hopkinson Press bar (SHPB) system at room temperature and after high-temperature treatment at 600 °C. The results show that the dynamic peak stress and energy consumption of rock were reduced by thermal treatment. The study identified the circular tensile- and compressive-tangential stress zones around the flaw boundary using elastic theory. The strain accumulation zones gradually shift from the tensile- to the compressive-tangential stress zones as the flaw inclination increases, resulting in a larger crack initiation angle and opening displacement, while also constraining the propagation path. Additionally, the high-temperature treatment strengthened the deformation in the circular tensile-tangential stress zone. In contrast to the severe failure of specimens at room temperature, the clear failure pattern of the specimens after high-temperature treatment merely consists of wing cracks. Finally, this study presents a detailed theoretical explanation of the cracking behavior under various conditions, which is consistent with the experimental results. This research enhances the understanding of dynamic crack propagation in deep high-temperature rocks and provides scientific references for engineering safety design and protection. [ABSTRACT FROM AUTHOR]